1. Field of the Invention
The present invention relates to a conductivity modulation type MOSFET of the trench type in which a gate is buried in a trench excavated in a semiconductor substrate.
2. Description of the Prior Art
A conductivity modulation MOSFET is also called an insulated gate bipolar transistor (IGBT). A sectional view of an IGBT of the planar type is shown in FIG. 2. A p base layer 3 is formed in the surface region of an n.sup.- base layer 2. Base layer 2 is formed on a p.sup.+ layer 1 which serves as a drain layer. n.sup.+ source layers 4 are oppositely disposed and formed in the p base layer 3. A p.sup.+ layer 5 under both the source layers 4 is also formed in the p base layer 3. In order to form a channel in the surface region of the p base layer 3 located between the source layer 4 and the n.sup.- base layer 2, a gate oxide layer 7 is formed on the structure, and a gate polycrystalline silicon layer 6 is formed on the gate oxide layer 7. A gate polycrystalline silicon layer 6 is connected to a gate terminal G through a gate electrode 8. A source electrode 10, which is connected to a source terminal S at an opening of a PSG insulating layer 9 covering the gate 6, is in contact with the p.sup.+ layer 5 and the n.sup.+ source layers 4. A drain electrode 11 connected to a drain terminal D is in contact with the drain layer 1 layered on the other side of the semiconductor substrate.
When a voltage larger than a fixed threshold value is applied to the gate electrode 8 with respect to the source electrode 10, the surface region of the p base layer 3 under the gate polycrystalline silicon layer 6 is inverted, creating a channel in the surface region of the p base layer 3. A conductive state then exists between the source S and the drain D. With hole injection through the channel, the n.sup.- base layer 2 is subjected to conductivity modulation increasing conductivity and allowing a large current flow. The value of this current is 10 to 20 times the value of current of an ordinary vertical power MOSFET without p.sup.+ drain layer 1. This is a highly advantageous feature.
The IGBT of the planar type shown in FIG. 2 inevitably suffers from the junction type field effect transistor (JFET) effect. In the JFET effect, a depletion layer is created by a diffusion potential arising from the junction between the n.sup.- base layer 2 and the p base layer 3. Consequently, the path for electrons is narrowed. Once conductivity modulation occurs, the JFET effect problem is removed. However, delay occurs in the rise of the voltage vs. current characteristic curve. In this respect, the characteristic curve rises unsmoothly.
A trench type IGBT which is free from the JFET effect and has a reduced on-resistance is proposed by H. R. Chang in IEDM 87 (International Electron Devices report, page 674). FIG. 3 shows a sectional view of this type of IGBT. In this transistor, a p base layer 3 sandwiches a trench 12. A gate polycrystalline silicon layer 6 is buried in the trench 12 with a gate oxide film 7 between the gate 6 and base layer 3. When a voltage is applied to the gate 6, a channel is formed along a layer perpendicular to the substrate surface 22 of the trench 12, in the p base layer 3 located between an n.sup.- base layer 2 and an n.sup.+ source layer 4 formed in the surface region of the substrate.
The operation of the element thus structured will be described with reference to FIG. 4, which shows an equivalent circuit of the element whose structure is shown in FIG. 2 or FIG. 3. The circuit of FIG. 4 contains a base short resistor Rp, a pnp transistor 32, and an npn transistor 33, in addition to a MOSFET 31. The pnp transistor 32 is made of the p base layer 3, the n.sup.- base layer 2, and the p.sup.+ drain layer 1 as shown in FIG. 2 or FIG. 3. The npn transistor 32 is made of the n.sup.+ source layer 4, the p base layer 3, and the n.sup.- base layer 2 as shown in FIG. 2 or FIG. 3. The base short resistor Rp represents a resistor that is present when the p base layer 3 and the p.sup.+ layer 5 are connected in series to the source electrode. In operation, a voltage higher than a threshold voltage is applied to the gate. Then, when the MOSFET 31 is turned on, electrons flow from the source to the base of the pnp transistor, and the entire element itself turns on.
In the IGBT shown in FIG. 2 or FIG. 3, the gate 6 and the source electrode 10 are insulated by the insulating layer 9. If this insulating layer has a manufacturing defect, the gate and the source will be shorted. In the trench IGBT, whose gate area is smaller than that of the planar IGBT, the probability of a short circuit is slightly reduced, but is very difficult to entirely eliminate the problem.